It is well known that class AB amplifiers are superior to transconductance amplifiers for efficiently providing high DC power to loads, due to the ability of class AB amplifiers to source and sink DC currents on demand. Transconductance amplifiers on the other hand have superior phase margins, which minimize the time lag between application of a differential voltage to the amplifier input and the resulting slew rate of the voltage at the amplifier output in response to the input signal. Phase margins of 90.degree. are typical for transconductance amplifiers, allowing operation at higher slew rates with limited signal distortion and overshoot.
Generally, circuit designers requiring amplifiers suitable for both high frequency operation and providing high DC currents must settle for higher signal distortion, lower power operation, or narrower operating bandwidths. For example, the addition of a pole-splitting compensation capacitor, C.sub.c, to a class AB amplifier that drives a capacitive load, C.sub.1, moves the second pole to a higher frequency, but compromises the unity gain bandwidth of the amplifier. To improve the phase margin, the second pole, g.sub.m2 /C.sub.1, must exceed the unity feedback frequency, g.sub.m1 /(Av C.sub.c) by the desired gain margin. The compensation capacitor limits the bandwidth over which the amplifier can operate in order to increase the capacitive loads that the amplifier can drive. For higher bandwidth, the amplifier must dissipate more power.
Transconductance amplifiers, while providing stable, fast response, may also translate a DC load current into an input offset voltage. Thus, there is a need for an amplifier circuit that can both provide power for DC loads and handle high frequency signals with minimal distortion.